The table contains some details about the polypeptides that form haemoglobin. Researchers examined the part of the $\beta$-globin polypeptide chain that includes the amino acid cysteine at position $93$. They discovered that: • this region points outwards when no oxygen is bound to the haem group • this region points inwards when oxygen is bound to the haem group • substituting cysteine with another amino acid lowers the Bohr shift. What conclusion can be drawn from the information about cysteine in haemoglobin?
- AMore than $1\%$ of the amino acids in one haemoglobin protein are cysteine.
- BIn $\alpha$-globin, there is a cysteine closer to the end of the polypeptide chain with an unreacted carboxyl group than in $\beta$-globin.
- CThe replacement of the cysteine at position $93$ in $\beta$-globin decreases the affinity of haemoglobin for oxygen at low pH.
- DThe binding of oxygen to the haem group causes the region of $\beta$-globin containing cysteine at position $93$ to become more hydrophilic.